A seal ring is provided to seal an annular gap between a relatively rotating shaft and a housing in order to hold oil pressure in an automatic transmission (AT) or a continuously variable transmission (CVT) in an automobile. There has been demand in recent years to reduce rotary torque in the above seal ring with advancements in lowering fuel consumption as a measure to address environmental issues. Accordingly, measures have conventionally been adopted for reducing the contact area of a sliding portion between the seal ring and a side wall surface of an annular groove where the seal ring is mounted. This type of seal ring will be described with reference to FIG. 8 in a conventional example.
FIG. 8 is a schematic cross-sectional view illustrating a seal ring according to the conventional example in use. The seal ring 300 according to the conventional example is mounted in an annular groove 510 provided around the outer circumference of a shaft 500. Further, the seal ring 300, in close contact with the inner peripheral surface of a shaft hole in the housing 600 where the shaft 500 is inserted, slidably contacts a side wall surface of the annular groove 510 and seals the annular gap between the shaft 500 and the housing 600.
Here, a pair of recessed sections 320 that extend in a circumferential direction is provided on the inner peripheral sides of both side surfaces on the seal ring 300 according to the conventional example. By this, an effective pressure receiving region, the region shown by A in FIG. 8, is produced when the seal ring 300 is pressed in an axial direction facing the low pressure side (L) from the high pressure side (H) by the sealed fluid. That is, of the side surfaces of the seal ring 300, the region in the radial direction of the portion 310 where there is no recessed section 320 makes up the effective pressure receiving region A. This is because fluid pressure acts on the region where the recessed sections 320 are provided from both sides in an axial direction thereby offsetting the applied forces in an axial direction with respect to the seal ring 300. Note that an area across the entire circumference of the pressure receiving region A makes up an effective pressure receiving area in an axial direction.
Moreover, an effective pressure receiving region, the region shown by B in FIG. 8, is produced when the seal ring 300 is pressed outward radially facing the outer peripheral surface side from the inner peripheral side by the sealed fluid. That is, the thick portion in the axial direction becomes the effective pressure receiving region B in the seal ring 300. Note that an area across the entire circumference of the pressure receiving region B makes up a pressure receiving area in a radial direction.
Accordingly, setting the “length of region A”<“the length of region B” allows sliding between the seal ring 300 and the side wall surface of the annular groove 510. Further, making the length of the pressure receiving region A to be as small as possible enables a reduction in rotary torque.
However, the contact region of the seal ring 300 against the side wall surface of the annular groove 510 is the region shown by C in FIG. 8. That is, with the seal ring 300, only the portion excluding the portion exposed to the gap between the shaft 500 and the housing 600 contacts the side wall surface of the annular groove 510 from the portion 310 where the recessed section 320 is not provided, being the side surface of the low pressure side (L). Therefore, the contact region C in the seal ring 300 is affected by the dimensions of the gap between the shaft 500 and the housing 600. Accordingly, depending on the use environment, the contact area of the seal ring 300 may be excessively small with respect to the side wall surface of the annular groove 510 thus risking lowering sealing performance. Further, according to the use environment, there is also a risk that the contact region may deform thereby destabilizing sealing performance. Additionally, as chamfer forms in the interface portion of the annular groove 510 and the outer peripheral surface of the shaft 500, the above problems are exacerbated as the chamfer increases.